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Photo by Karen Dixon. Photo by Karen Dixon.
Figure 20. Lack of positive guidance at a driveway. Figure 21. Object orientation with access ramp.
vehicles toward roadside objects. Figure 21 shows one crash Roadside Configurations Commonly Known
location where this occurred. This crash condition is similar to Be Hazardous
to the driveway crash condition with no positive guidance
Several roadside crashes occurred at locations where they
discussed previously.
would be expected. These sites exhibited characteristics known
to result in potentially hazardous conditions. For example,
Unique Roadside Configurations Associated locations with roadside ditches, nontraversable headwalls and
with High Crash Occurrence culverts (often at driveways), or uneven roadside grading were
common roadside crash locations. In addition, crashes
Several of the study corridors were characterized by high
occurred at high-speed locations where sloping curb delin-
crash numbers at a specific location. Often this peak in crash
eated the roadside edge, but adequate clear zone was not
statistics resulted from a physical road feature unique to the
available. Finally, three of the corridors were located in the
site. For example, at corridor UCZ-IL-1 a disproportionately
vicinity of scenic or tourist attractions. At these locations,
large number of crashes involved an underpass structure.
roadside objects were hit more frequently even when lateral
When members of the research team inspected the site, they
offsets to the objects were similar to those at other crash-free
determined that sometime in its past, a two-way road with
sites. Regardless of the cause, additional lateral offset to
one lane under each side of the underpass wall had been
objects in these or similar locations seems prudent to mini-
converted into a two-lane, one-way road. This modification
mize the risk of hazardous run-off-road crashes for unfamiliar
occurred at two separate locations due to the creation of a
drivers.
one-way pair configuration. As a result, the approach to the
underpass required vehicles to shift in an effort to avoid the
wall now located between the lanes in the same direction of Case Study Task and
travel. The crashes occurred when a vehicle did not safely Summary of Findings
navigate this required lane shift. This crash cause was evident
Experimental Design
due to a scarred underpass wall.
Locations of this type are unique and should be considered The individual projects selected as part of this case study
individually for crash mitigation treatments. The creation of task were used to identify strategies where safety and aes-
simple crash spot maps (as shown in Appendix A) can enable thetics were incorporated into the roadway's design. The
an agency to quickly identify cluster crash locations of this research team identified several recent beautification or
nature. roadside improvement projects to use as indicators for the
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influence of improvements on crash conditions. Ideally, 3. Crash severity--the proportion of crashes involving seri-
a project where only one item is changed (such as moving ous injury or death compared with property-damage-only
trees to the far side of sidewalks) would be perfect for this (PDO) crashes.
task; however, the research team could not identify projects 4. Crash type--changes in specific types of crashes that have
of this nature because, in general, transportation agencies occurred as a result of the improvement.
implement multiple improvements in each project. As a re- 5. Average daily traffic (ADT)--the average number of ve-
sult, the data analysis for this case study task can provide hicles per day using the roadway.
general indications about the safety impacts of beautifica-
tion or roadside improvement projects, but cannot be used In addition to crash type summary information and crash
to explicitly evaluate individual features and their associated severity summary information, Appendix B includes a simple
hazards. before-after crash summary comparison for each site. Often
Initially, the research team proposed two levels of case before-after analyses are limited because researchers study
studies (one with comparison sites); however, the NCHRP corridors where safety issues are prominent and so the re-
Project 16-04 panel requested the stand-alone case studies sulting improvements can be dramatic; however, for the beau-
(locations without comparison sites) so as to evaluate as tification and roadside enhancement projects the focus is not
many projects as possible. The stand-alone case studies in- on operations and safety but rather on aesthetics and livability.
clude crash type summaries, crash severity summaries, and As a result, the before-after analysis can provide a useful in-
before-after crash analysis for a specific improvement dication about possible safety implications of a change to the
corridor with data developed and provided by local juris- road environment. Table 21 demonstrates the basic type of
dictions. The research team attempted to solicit candi- data included in the before-after analysis for each case study
date projects from a variety of geographically distributed included in Appendix B.
jurisdictions.
Inclusion of a case study project required that the project
Findings and Recommendations
have a focus on median or roadside improvements as well as
having available most, if not all, of the requested data. The Table 22 illustrates the individual case study elements in-
research team attempted to collect crash data for 3-year cluded in Appendix B as well as the general observed safety
periods before and after the project was implemented. This trend for each project. The research team attempted to exclude
level of crash information was not available for all sites. projects in which entire lanes were added as the observed
In select cases, projects with a minimum of 1 year of data safety results because these types of projects provide con-
were included; projects with less than 1-year's worth of founding information; nonetheless, a few of the projects had
post-reconstruction data were excluded. The construction some lane widening (often due to realignment) and are so
period is indicated in the case study summaries included in noted in the table. The crash trends identified in Table 22
Appendix B. show (1) when crash frequency increased by more than one
More specifically, the research team collected data in the crash per year (or by more than 5 percent), (2) when crash
following five areas for the case study analysis: frequency decreased by more than one crash per year (or by
more than 5 percent), or (3) when change in crash frequency
1. Crash frequency--the absolute number of crashes occur- was minimal (within one crash per year on average or within
ring before and after the context-sensitive improvement. 5 percent of the original crash rate). These crash trends are a
2. Crash rates--raw crash volumes considered in relation to summary of the before-after analysis documented in Table 21
the traffic volume carried by the roadway. and the individual case studies included in Appendix B.
Table 21. Evaluation matrix for case study sites.
Comparison
Candidate Street -- Candidate Street -- Crash Standard
Analysis Category Before After Reductions Deviation
Crash Frequency
Crash Rate
Severe and Fatal
Crash Frequency
Single-Vehicle
Crash Frequency
ADT
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Table 22. Case study project elements versus before-after crash trends.
Streetscape Project Elements Before-After Crash Trends*
Widening of Road > 8'
Add or Enhance Street
Relocate Utility Poles
Sidewalk Additions /
Frequency of Single-
Enhanced Pedestrian
Frequency of Severe
Add Landscaping /
Crossings / Access
Improve Roadside
Landscape Buffer
Frequency of All
Bus Stops / Bays
Median Islands /
Curb Extensions
Trees Removal /
Vehicle Crashes
Next to Road &
Curb & Gutter
Raised Islands
Grading/Ditch
Improvements
Bicycle Lanes
Street Trees
Crash Rate
Relocation
Sidewalk
Removal
Crashes
Crashes
Lights
Case No.
CS-AZ-1 x x x x x x x
CS-AZ-2 x x x x x x x
CS-AZ-3 x x x x
CS-CA-1 x x x x x
CS-CA-2 x x x x
CS-CA-3 x x x x x x x x
CS-MN-1 x x x x x x
CS-MT-1 x x x
CS-MT-2 x x x x x
CS-NC-1 x x x x x
CS-NC-2 x x x x x x x x
CS-NC-3 x x x
CS-NC-4 x x x x x x x
CS-NC-5 x x x x x
CS-NC-6 x x x x x x x
CS-NC-7 x x x
CS-OR-1 x x x x x
CS-OR-2 x x x x x x x
CS-OR-3 x x x
CS-OR-4 x x
CS-OR-5 x x x x x x
CS-OR-6 x x x x
CS-OR-7 x x x x x
CS-UT-1 x x x
CS-UT-2 x x
CS-UT-3 x x x
CS-UT-4 x x x
*Before-After symbols depict the following:
Crash frequencies increased by more than one crash per year; crash rates increased by more than 5 percent.
Crash frequencies decreased by more than one crash per year; crash rates decreased by more than 5 percent.
Crash frequencies for the "After" condition were within one crash per year of the "Before" condition; crash rates for the "After" condition
were within 5 percent of the "Before" condition crash rates.
In Table 22, the before-after crash trends are represented Only three of the case study sites exhibited an increase greater
by the four statistics: than one additional severe crash per year. All three of these
case study sites included sidewalk improvements with buffer
· Frequency of all crashes at a site, strips, but several similar improvement projects resulted in
· Crash rate, little change to a reduction in severe crashes.
· Frequency of severe crashes at a site, and Since the focus of this research effort is roadside crashes,
· Frequency of single-vehicle crashes. and these frequently are single-vehicle crashes, an increase in
these kinds of crashes may be of concern. Single-vehicle
Ideally, a reduction in all four trend statistics would be crashes increased by more than one crash at eight of the sites.
observed, clearly demonstrating enhanced safety at a site; In general, these sites included pedestrian enhancement im-
however, in many cases, an increase occurred for one before- provements; however, as was the case with the sites of severe
after crash trend statistic while others remained constant or crashes discussed above, there were many pedestrian enhance-
decreased. For all candidate improvement projects, a designer ment projects that resulted in reduced single-vehicle crashes.
seeks to reduce the number of severe crashes at a site. Severe Since inspection of the individual before-after crash trends
crashes, for the purposes of the values shown in the case study provides confounding results, a more effective approach may
tables, generally include incapacitating injuries or fatalities. be to examine all four before-after crash trends collectively.
